Residence time study of a two-phase mixture in a horizontal cylindrical vessel

A cold model of a rotary holding furnace was studied using water and a kerosene-LIX® 973N organic mixture which are immiscible. The flow of the feed was found to behave similar to a gravity current where the feed preferentially moved along the liquid–liquid interface. Visual observation and residence time distribution obtained showed that the flow of the lighter feed mixture was similar to a laminar flow but with a preferential route along the wall with the outlet spout. In the commercial-scale rotary holding furnace, plug flow conditions are considered desirable while mixing or short-circuiting is considered undesirable. The flow in the cold model was fitted to a plug flow and three CSTRs all in series and a particular depth of the upper layer of organic was found where mixing with the bath fluid was a maximum. Air bubbling in the centre of the cold model showed that at low air flow rates, the air curtain acted to limit mixing but as the air flow rates increased, the increased circulation caused by the air flow increased mixing and negated the air curtain effect.

► We modelled the flow of a two immiscible liquid mixture in a cold model of a horizontal cylindrical separator.
► It showed that the flow was similar to a gravity current at the interface of the two liquids.
► The residence time distrubtion was measured and the flow was modelled with a plug and CSTR combination.
► The use of gas bubblers in the separator can lead to compartmentalisation at low gas flow rates.
► Maximum mixing was found to occur at a particular depth of the upper liquid layer which should be avoided if good separation is to be achieved.